1. Field of the Invention
This invention relates to a hydraulic circuit system for use in swivel type excavators, and in particular to a hydraulic circuit system for controlling variable displacement pumps so as to reduce hydraulic fluid flow delivered by the pumps to minimum when operation control valves for controlling manipulations of work implements are located at their respective neutral positions.
2. Description of the Prior Art
There have so far been proposed several hydraulic circuit systems such as, for example, Japanese patent application No. 51536/1972 in which, for the purpose of achieving an energy saving, control of variable displacement pumps can be made so as to reduce the flow rate of hydraulic fluid delivered thereby to minimum when work implements are not operated. However, the above-mentioned prior art hydraulic circuit system has been disadvantageous in that, when swivel operation control valves are operated to swivel the upper swivel body, hydraulic fluid flow delivered by the variable displacement pumps tends to be compensated with one another, so that a proper flow rate of hydraulic fluid required for swivelling the upper swivel body cannot be obtained.